Measurements of the amount of steady-state hnRNA and parameters characterizing its size distribution may be of value in understanding the biological properties of malignant cells. The populations of hnRNA found in several transformed and normal 3T3 cells and one normal and transformed human fibroblast line were compared. Uncompleted studies indicate that although the human and murine cells have characteristically different hnRNA populations, the normal and transformed cell lines are very similar.